Oecologia (2000) 122:129–137 © Springer-Verlag 2000 Andreas Kruess · Teja Tscharntke Species richness and parasitism in a fragmented landscape: experiments and field studies with insects on Vicia sepium Received: 4 January 1999 / Accepted: 8 September 1999 Abstract Effects of habitat fragmentation on species di- Introduction versity and herbivore-parasitoid interactions were ana- lyzed using the insect community of seed feeders and Habitat fragmentation following increasing intensity of their parasitoids in the pods of the bush vetch (Vicia se- land use in the landscape (Burgess and Sharpe 1981) has pium L.). Field studies were carried out on 18 old mead- been perceived as a major threat to biological diversity ows differing in area and isolation. The area of these (Wilcove et al. 1986; Noss 1991; Saunders et al. 1991; meadows was found to be the major determinant of spe- Tscharntke 1992; Rosenzweig 1995). The effects of hab- cies diversity and population abundance of endophagous itat fragmentation on species diversity can be mainly as- insects. Effects of isolation were further analyzed experi- signed to three processes: reduction of total habitat area mentally using 16 small plots with potted vetch plants within a region, loss of area within each single habitat, isolated by 100–500 m from vetch populations on large and increase in isolation between habitats (Andrén old meadows. The results showed that colonization suc- 1994). These three processes are interrelated in a non- cess greatly decreased with increasing isolation. In both linear way (Gustavson and Parker 1992). Theoretical cases, insect species were not equally affected. Parasito- models such as the equilibrium theory of island biogeo- ids suffered more from habitat loss and isolation than graphy (MacArthur and Wilson 1967) predict the number their phytophagous hosts. Minimum area requirements, of species on islands as a function of island size and iso- calculated from logistic regressions, were higher for lation. Loss of species may lead to changes in ecosystem parasitoids than for herbivores. In addition, percent para- functions such as decomposition, pollination, parasitism, sitism of the herbivores significantly decreased with area or predation (Kareiva 1987, 1990; Klein 1989; Kruess loss and increasing isolation of Vicia sepium plots, sup- and Tscharntke 1994, 1999; Didham et al. 1996; Burkey porting the trophic-level hypothesis of island biogeogra- 1997; Steffan-Dewenter and Tscharntke 1997; Didham phy. Species with high rates of absence on meadows and 1998; Dubbert et al. 1998; Tscharntke and Kruess 1999). isolated plant plots were not only characterized by their Ecosystem functions based on trophic structure were the- high trophic level, but also by low abundance and high oretically analyzed by Holt (1996), who investigated the spatial population variability. Thus conservation of large general effects of area loss and isolation on food chain and less isolated habitat remnants enhances species di- length and species of different trophic rank. He predicted versity and parasitism of potential pest insects, i.e., the that (1) food chain length should increase with area and stability of ecosystem functions. decrease with isolation, and (2) that the slope of species- area and species-isolation curves should increase with Key words Island biogeography · Insect diversity · trophic rank, either due to direct effects (e.g., decline of Herbivore-parasitoid interactions · Trophic levels · population size with trophic rank) or indirect effects (de- Biological control pendence of high-rank species on the presence of low- rank species). Not only trophic position but also feeding type may be differentially influenced by fragmentation, and several studies have examined the effects of fragmen- tation on both ectophagous (e.g., Davis 1975; Ward and Lakhani 1977; Rigby and Lawton 1981; Tscharntke 1992; A. Kruess (✉) · T. Tscharntke Matter 1997; Zabel and Tscharntke 1998) and endophag- Agroecology, University of Göttingen, Waldweg 26, D-37073 Göttingen, Germany ous insect communities on plants (MacGarvin 1982; e-mail: [email protected] Davis and Jones 1986; Tscharntke 1992; Kruess 1996; Tel.: +49-551-392359, Fax: +49-551-398806 Dubbert et al. 1998). 130 Table 1 List of landscape, meadow and host plant charac- Field studies on meadows Field experiment with Vicia sepium plots teristics used in multiple regres- sion analyses Landscape characteristics Landscape characteristics Number of meadows within a radius of 250 m Total area of the “source meadow” (m2) Number of meadows within a radius of 500 m Distance to the source meadow (m) Total area of meadows within a radius of 250 m (m2) Distance to the nearest near-natural habitatc (m) Total area of meadows within a radius of 500 m (m2) Distance to the nearest fallow (m) Distance to the nearest meadow (m) Distance to the nearest hedge (m) Distance to the nearest meadow (>2 ha) (m) V. sepium plants m–2 on source meadows V. sepium pods m–2 on source meadows Meadow characteristics Meadow size (m2) Plant plot characteristics Center/edge ratioa Mean plant height (cm) Number of vascular plant species per 49 m2 Number of plants m–2 Cover of vegetation (%)b Number of pods m–2 Mean vegetation height (cm) Mean pod length (cm) Mean number seeds pod–1 Host plant characteristics Number of legume species a Edge was defined as the 5 m Cover of legume species (%) wide margin. Center/edge ratio Number of Vicia species was calculated from the center Cover of Vicia species (%) area (m2) and the edge area Cover of V. sepium (%) (m2) Pods m–2 of V. sepium b Estimated by eye Mean pod length of V. sepium (cm) c Including hedges, forest Mean seeds pod–1 of V. sepium edges, and fallows We analyzed the effects of fragmentation on the endo- We addressed the following questions: phagous insects inhabiting the pods of bush vetch (Vicia 1. Are species richness and abundance affected by habi- sepium), including herbivores and parasitoids. Since en- tat area, habitat isolation, or other landscape, mead- dophagous insect communities are more likely to com- ow, or host plant characteristics? prise specialized mono- or oligophagous species than 2. Are parasitoids more affected than herbivores? ectophagous insects (Cornell 1989), area and isolation of 3. Does higher susceptibility of parasitoid species lead habitats can be defined by the distribution of the host to a reduction in parasitism? plant populations. Moreover, plant-insect systems with 4. How can species that are susceptible to habitat frag- endophagous species seem to be more likely to show mentation be characterized? changes in diversity and abundance due to environmen- tal changes since they are more host-specific and less mobile (Cornell 1989). Previous experimental studies on the effects of isola- Material and methods tion on colonization success of endophagous insect her- Field samples were taken in a region called Kraichgau, situated bivores and their insect parasitoids on red clover (Kruess near Karlsruhe in south-west Germany. In the first half of this cen- and Tscharntke 1994, 1999; Kruess 1996) found that tury, this landscape was dominated by extensively used meadows food-web interactions were disrupted: on isolated plant with scattered standard cherry and apple trees. Most of these patches, parasitoid populations were more reduced than meadows had areas of several hectares, often surrounding villages like “green belts”. Intensification of agricultural land use since the their phytophagous hosts. In this paper, results are pre- 1930s has led to a dramatic loss and fragmentation of these mead- sented from field samples and from a field experiment ows. Nowadays, most of the remaining meadows are only small on the endophagous insect community in the pods of an- fragments of less than 1 ha. Few meadows of more than 10 ha still other legume, the bush vetch (Vicia sepium). Bush vetch exist. Within a region of 400 km2, 18 meadows with areas ranging 2 is a very abundant and widely distributed plant species in from 300 m to 70 ha were selected to analyze the effect of habitat area. Selection criteria were similarity in age, management, vege- old meadows in south-west Germany. The pods are easi- tation, exposition, and soil type (Loess). All meadows were at ly collected and rearing success of the insects is high. least 30 years old and were mown annually in mid-July. They Regional data on the range of host plants and hosts of the were scattered with apple trees and slightly exposed to the south expected insects were available from Garbe (1996), who or south-east. In June, vegetation of the meadows was mapped in 49-m2 plots (1×49 m2 on meadows <1000 m2, 2×49 m2 on mead- investigated the endophagous herbivores inhabiting the ows of 1000–10,000 m2, and 4×49 m2 on meadows >10,000 m2). pods of 27 leguminous plant species in the Kraichgau re- The parameters used to characterize the landscape surrounding the gion. Field samples were taken on extensively managed meadows, the meadows and the host plants and considered in the old meadows, to analyze the effects of habitat character- statistical analyses are shown in Table 1. From 3 to 6 July, 200 pods of the bush vetch V. sepium were collected randomly from istics (e.g., area, vegetation, surrounding landscape). The each meadow. The field experiment comprised 18 plots of potted field experiment tested the effects of isolation using V. sepium plants, each consisting of 12 pots, covering an area of small and isolated plots of potted vetches. 1.0 m2. Since this experiment was carried out to analyze the ef- 131 fects of isolation on colonization processes, we used five old Insect community meadows (area >2 ha) with naturally growing V. sepium as “sourc- es”. One plant plot was placed in the center of each meadow (con- trols), and the other 13 plant plots were established in the same lo- The endophagous insects found in the pods of V.